What is 'Quality Score' and how is it calculated?

clipped from adwords.google.com The AdWords system calculates a 'Quality Score' for each of your keywords. It looks at a variety of factors to measure how relevant your keyword is to your ad text and to a user's search query. A keyword's Quality Score updates frequently and is closely related to its performance. In general, a high Quality Score means that your keyword will trigger ads in a higher position and at a lower cost-per-click (CPC). influencing your keywords' actual cost-per-clicks (CPCs) estimating the first page bids that you see in your account determining if a keyword is eligible to enter the ad auction that occurs when a user enters a search query affecting how high your ad will be ranked